1. Field of the Invention
The present invention relates to a heater and a catalytic converter both having a resistance adjusting function and employing a honeycomb structure.
Honeycomb heaters of the above-described type can be employed as heaters for domestic use, such as hot air heaters, or as industrial heaters, such as preheaters used for control of automobile exhaust emission. The above-described catalytic converters can be applied for use in automobile exhaust emission control.
2. Description of the Related Art
Conventionally, porous ceramic honeycomb structures have been employed as catalysts or carriers for catalysts for removing, for example, nitrogen oxides, carbon monoxide and hydrocarbons present in the exhaust gas of internal combustion engines, such as automobiles, or filters for removing fine particles.
Whereas porous ceramic honeycomb structures continue to be a popular and useful material in such environments, there has been a desire to develop materials exhibiting greater mechanical strength and thermal resistance in hostile environments.
Apart from the above honeycomb structures, as restriction of exhaust emission has been intensified, there has been a demand for development of heaters for use in automobile exhaust emission control.
Honeycomb structures have been proposed in, for example, U.S. Pat. No. 4,758,272, Japanese Utility Model Laid-Open No. 67609/1988 and U.K. Patent 1492929.
The honeycomb structure disclosed in U.S. Pat. No. 4,758,272 has a composition essentially consisting, as analyzed in weight percent, of 5 to 50% Al, 30 to 90% Fe, 0 to 10% Sn, 0 to 10% Cu, 0 to 10% Cr and no more than 1% Mg and/or Ca. This honeycomb structure has a porosity of 25 to 75% and a predetermined cell density, and is used as a diesel particulate filter.
However, U.S. Pat. No. 4,758,272 does not disclose the use of the above-described honeycomb structure as a heater or a catalytic converter.
U.K. Patent 1492929 discloses the use of a foil type metal honeycomb structure in a catalyst for use in automobile exhaust emission control. This honeycomb structure comprises a metal substrate produced by winding, together with a flat plate, a mechanically deformed, corrugated flat plate. This metal substrate has an oxide aluminum film formed on the surface thereof by the oxidation process. The catalyst for use in automobile exhaust emission control is manufactured by placing a high surface area oxide, such as alumina, on the oxide aluminum film of the metal substrate and by supporting a noble metal on the high surface area oxide.
Japanese Utility Model Laid-Open No. 67609/1988 discloses the use as a preheater of an electrically conductable metal monolith catalyst comprising a metal support and alumina coated thereon.
In the foil-type metal honeycomb structure disclosed in U.K. Patent 1492929, however, the metal substrate with a coating formed thereon cannot be closely adhered to a catalyst layer because of its low porosity, and a ceramic catalyst readily peels off the metal substrate due to a difference in the thermal expansion between the ceramic catalyst and the metal substrate. Furthermore, a telescope phenomenon readily occurs during the run cycle in which a metal-to-metal joint breaks and the metal substrate is deformed in such a manner that it protrudes in the direction of the flow of gas. This may disturb safe running of the vehicle. Furthermore, in the manufacture of the foil type metal honeycomb, yield of the rolling process is low, inviting high production costs. In the preheater proposed in Japanese Utility Model Laid-Open No. 67609/1988, coated alumina readily peels off a metal support due to a difference in thermal expansion between alumina and the metal support. Furthermore, a metal-to-metal joint of the metal substrate breaks during the operation, generating an electrically insulating portion and, hence, non-uniform flow of current and non-uniform heating.
The preheater disclosed in Japanese Utility Model Laid-Open No. 67609/1988 is constructed such that a current is supplied between the inner periphery and the outer periphery of the foil type metal honeycomb structure to generate heat. However, the preheater is not arranged such that it has an adjusted resistance (that is, the material, dimension and rib thickness of the honeycomb structure define the resistance but a desired resistance cannot be adjusted), and therefore exhibits insufficient temperature rising characteristics. Furthermore, since the electrodes are provided on the inner peripheral portion of the preheater, the central portion thereof does not act as a catalyst and pressure loss may be generated. Furthermore, the electrodes readily break due to the flow of gas.